1. Field of the Invention
The present invention relates to graphite electrodes for electrothermic reduction furnaces, in particular for the production of aluminum, titanium, silicon, ferroalloys, phosphorous. The invention also pertains to a method of producing such graphite electrodes.
2. Description of the Related Art
For a century the aluminum industry has relied on the Hall-Heroult process for aluminum smelting. In comparison with processes used to produce competing materials, such as steel and plastics, the process is energy-intensive and costly. Hence, alternative aluminum production processes have been sought.
One such alternative is the process referred to as direct carbothermic reduction of alumina. As described in U.S. Pat. No. 2,974,032 (Grunert et al.) the process, which can be summarized with the overall reactionAl2O3+3C=2Al+3CO  (1)
takes place, or can be made to take place, in two steps:2Al2O3+9C=Al4C3+6CO  (2)Al4C3+Al2O3=6Al+3CO  (3).
Reaction (2) takes place at temperatures between 1900 and 2000° C. The actual aluminum producing reaction (3) takes place at temperatures of 2200° C. and above; the reaction rate increases with increasing temperature. In addition to the species stated in reactions (2) and (3), volatile Al species including Al2O are formed in reactions (2) and (3) and are carried away with the off gas. Unless recovered, these volatile species represent a loss in the yield of aluminum. Both reactions (2) and (3) are endothermic.
Various attempts have been made to develop efficient production technology for the direct carbothermic reduction of alumina (cf. Marshall Bruno, Light Metals 2003, TMS (The Minerals, Metals & Materials Society) 2003). U.S. Pat. No. 3,607,221 (Kibby) describes a process in which all products quickly vaporize to essentially only gaseous aluminum and CO, containing the vaporous mixture with a layer of liquid aluminum at a temperature sufficiently low that the vapor pressure of the liquid aluminum is less than the partial pressure of the aluminum vapor in contact with it and sufficiently high to prevent the reaction of carbon monoxide and aluminum and recovering the substantially pure aluminum.
Other patents relating to carbothermic reduction to produce aluminum include U.S. Pat. Nos. 4,486,229 (Troup et al.) and 4,491,472 (Stevenson et al.). Dual reaction zones are described in U.S. Pat. No. 4,099,959 (Dewing et al.). More recent efforts by Alcoa and Elkem led to a novel two-compartment reactor design as described in U.S. Pat. No. 6,440,193 (Johansen et al.).
In the two-compartment reactor, reaction (2) is substantially confined to a low-temperature compartment. The molten bath of Al4C3 and Al2O3 flows under an underflow partition wall into a high-temperature compartment, where reaction (3) takes place. The thus generated aluminum forms a layer on the top of a molten slag layer and is tapped from the high-temperature compartment. The off-gases from the low-temperature compartment and from the high-temperature compartment, which contain Al vapor and volatile Al2O are reacted in a separate vapor recovery units to form Al4C3, which is re-injected into the low-temperature compartment. The energy necessary to maintain the temperature in the low-temperature compartment can be provided by way of high intensity resistance heating such as through graphite electrodes submerged into the molten bath. Similarly, the energy necessary to maintain the temperature in the high-temperature compartment can be provided by a plurality of pairs of electrodes substantially horizontally arranged in the sidewalls of that compartment of the reaction vessel.
With the exception of aluminum production, electrothermic reduction of various metals and also non-metals, such as titanium, silicon, ferroalloys, as well as phosphorous, are well-established industrial processes. Due to the relatively low current densities, ranging from 6 to 10 A/cm2, in many of these processes self-baking carbon electrodes (also called “Söderberg electrodes”) are being used.
The use of self-baking carbon electrodes has been known for a long time (see U.S. Pat. Nos. 1,440,724 and 1,441,037 to Söderberg). Self-baking carbon electrodes basically consist of a pasty mixture of carbon-containing material such as anthracite, coke, tar, and pitch, which is filled into a metal casing held in position within an electric arc furnace by way of contact shoes and a suspension/sliding device. The application of high electric currents plus the heat of the arc struck by the electrode during the furnace operation develops sufficient heat to melt the material filled into the casing and form a paste, then cokify the so-formed paste, and finally bale the electrode. In accordance with its consumption rate the electrode is lowered stepwise, a new casing sheet is joined to the upper part, the casing is filled with the mixture, and the middle section is baked. In a variation, the electrode may be partly baked at a low temperature of about 600° to 700° C. In the context of the Söderberg electrode, the lower part of the steel casing dissolves in the bath of molten metal, thus injecting iron into the bath. To avoid this contamination by iron, several solutions have been proposed, which all consist of mechanically detaching the electrode and the steel casing so that the electrode can be caused to slide without the steel casing.
U.S. Pat. No. 6,635,198 (Vatland et al.) describes a method for the continuous production of self-baking composite electrodes utilizing sectioned metallic casings. Each new section of casing is mounted upon the section of casing below without applying welding or other means to rigidly affix the section to each other. As the sections of casing are not rigidly affixed to each other by welding or the like, it is easy to remove the casing after the electrode has been baked.
Another solution is a mounting configuration as described in U.S. Pat. No. 4,575,856 (Persson) which involves supporting the weight of the electrode by means of a column formed from pre-baked carbon or graphite electrodes being enclosed by the baked paste, both the column and the paste being consumed at the same time.
Modern electric arc furnaces for steel production are operated at current densities in excess of 25 A/cm2 and thus require highly conductive graphite electrodes. To achieve electrical resistivities below 10 μOhm m, such graphite electrodes are produced using well-ordered needle cokes and they are graphitized at temperatures above 3000° C. The use of costly needle coke and the high electricity costs for graphitization bar such electrodes from being used in low-power electric furnaces that are used for producing non-steel materials. Furthermore, iron oxides are added to the electrode raw material mixture to inhibit puffing (caused by the release of sulfur from its bond with carbon inside the coke particles). Hence, the increased iron content can contaminate the melt and cause high electrode erosion in melt furnace atmospheres that are rich in CO, such as in the case of carbothermic reduction of alumina.